O.H. Hughes

Sequential tunneling due to intersubband scattering in double‐barrier resonant tunneling devices

Magnetoquantum oscillations in the tunnel current of double‐barrier n‐GaAs/(AlGa)As/GaAs/(AlGa)As/GaAs resonant tunneling devices reveal evidence of sequential tunneling in the voltage range corresponding to the resonance when electrons tunnel into the second subband of the GaAs quantum well. The sequential tunneling arises from intersubband scattering between two quasi‐bound states of the well. Near this resonance, the charge buildup in the well can be estimated from the magnetoquantum oscillations.

Electrical and spectroscopic studies of space-charged buildup, energy relaxation and magnetically enhanced bistability in resonant-tunneling structures

Abstract Photoluminescence measurements and magnetoquantum oscillations in the differential capacitance are used to measure space-charged buildup and study electron thermalization in a double-barrier resonant tunneling structure based on n-type (AlGa)As. The intrinsic bistability observed in the I(V) characteristics is also seen in the linewidth and photon energy of the photoluminescence. The spectroscopic data reveal clearly the importance of intersubband transitions in the voltage range at which electrons tunnel resonantly into the second bound state of the quantum well. A novel field-induced enhancement of the intrinsic bistability effect is reported for B ‖ J .

Observation of space-charge bulk-up and thermalisation in an asymmetric double-barrier resonant tunnelling structure

By means of a study of magnetoquantum oscillations in the differential capacitance, the authors have observed the thermalisation of the space charge stored dynamically in the quantum well of an asymmetric double-barrier resonant tunnelling heterostructure based on n-GaAs/(AlGa)As. Fourier analysis of the oscillations was used to monitor the charge build-up in both the emitter accumulation layer and in the well. The storage time of an electron in the well was found to be approximately=0.5 mu s. The resonant tunnelling is truly sequential rather than coherent.

Charge build-up and intrinsic bistability in an asymmetric resonant-tunnelling structure

Intrinsic bistability is observed in an asymmetric resonant-tunnelling structure based on n-GaAs/(AlGa)As, incorporating a thin emitter barrier and a thick collector barrier. The resonant charge build-up in the quantum well which gives rise to the bistability is monitored by the Landau level structure in the magneto-capacitance.

Nitrogen species from radio frequency plasma sources used for molecular beam epitaxy growth of GaN

We have made a detailed study of the optical spectroscopy of two different RF plasma sources used for the growth of GaN by molecular beam epitaxy. Our studies show that for both sources the predominant species present in the cavity are nitrogen atoms. The strongest optical emission occurs at 869 nm. We have also studied, in detail, the factors which influence the ion content of the flux. Two key parameters are the temperature of the wall of the cavity and the size of the holes in the aperture plate from which the species emerge into the vacuum. We have identified conditions under which the ion content can be made negligibly small and show that this results in films with improved optical properties.

Gallium-induced surface reconstruction patterns of GaN grown by molecular beam epitaxy

Abstract We report on an investigation of the mechanisms giving rise to surface reconstruction for GaN grown by molecular beam epitaxy (MBE) on a range of different substrates. We have studied the effects of surface contamination by oxygen or arsenic and demonstrate that both can influence the surface reconstruction. We show that surface reconstruction measured by reflection high-energy electron diffraction (RHEED) is associated with excess Ga on the surface, which undergoes an order–disorder transition at about 400–500°C. For MBE, growth on hydride vapour phase epitaxy (HVPE) GaN/SiC composite substrates having the Ga polarity the 2×2 reconstruction is intrinsic. This intrinsic reconstruction can be destroyed by heating to a high temperature or by oxidation. The intrinsic 2×2 reconstruction can also be destroyed by adding an additional monolayer of more weakly bound Ga, which can be removed by desorption at high temperature. For growth by MBE on sapphire with nitridation, a 3×3 reconstruction is observed on cooling the sample to

The oscillatory magnetoresistance of electrons in a square superlattice potential

The transverse magnetoresistance of a two-dimensional electron gas in an n-type GaAs/(AlGa)As heterostructure subjected to a square superlattice potential is investigated. Magneto-oscillations are observed at low field (B ≤ 0.4 T) with period Δ(1/B) = ea/2kF, where a = 145 nm is the superlattice constant. At higher fields the magneto-resistance is dominated by Shubnikov-de Haas oscillations. A comparison is made with experiments on a one-dimensional superlattice.

Zn diffusion-induced disorder in AlAs/GaAs superlattices

Zn diffusion into AlAs/GaAs superlattices has been studied experimentally using transmission electron microscopy and secondary ion mass spectroscopy. It has been found that microdefects are present near the Zn-diffusion front. Analysis of these defects has shown them to be interstitial dislocation loops. It is believed that the diffusion of Zn into AlAs/GaAs multiquantum wells and superlattices is similar to that of Zn into GaAs. This provides evidence for the interstitial mechanism for the enhancement of the interdiffusion of the GaAs/AlAs superlattices.

Changeover from acoustic to optic mode phonon emission by a hot two-dimensional electron gas in the gallium arsenide/aluminium gallium arsenide heterojunction

The authors have used heat pulse techniques to study the energy relaxation of a hot two-dimensional electron gas (2 DEG) in a GaAs/AlGaAs heterojunction. The 2 DEG was heated by applying short ( approximately=100 ns) electrical pulses to the drain-source contacts of the device. The electrons lost energy by emitting phonons which were detected by a CdS bolometer on the opposite side of the GaAs substrate. A change in the nature of the phonon signal occurring at an excitation level of about 5 pW per electron indicated a change in the phonon emission process. The corresponding electron temperature, Te, at which optic phonon emission is expected to become the dominant energy relaxation process was estimated to be about 60 K. At powers well below the change-over, the authors found that the energy loss rate per electron, Pe, due to acoustic phonon emission is proportional to Te3. At much higher powers, Pe varies as exp(-h(cross) omega LO/kTe), where h(cross) omega LO is the longitudinal optic phonon energy. They obtained a value of 3.3 ps for the electron-optic phonon scattering time, which is consistent with the range of values found in the literature.

Hybrid magneto-electric states in resonant tunnelling structures

Double barrier resonant tunnelling structures with wide undoped quantum wells are used to study quantum ballistic transport in the presence of a magnetic field B. The structures are based on n−GaAs/(AlGa)As with well widths of 60 and 120 nm. At B=0, the wider well structure (120 nm) shows as many as 70 resonances in I(V). With B applied in the plane of the barriers (B·J) these resonances evolve into hybrid magneto-electric states. At sufficiently large B, the electron orbits no longer extend to the second barrier and tunnelling occurs into cycloidal interface states which are localised near the emitter barrier. A theoretical model for the observed resonances based on the quantisation of the hybrid and cycloidal orbits is presented. Ballistic path lengths of at least 400 nm are observed.